SAN FRANCISCO -- Apple Computer Inc. yesterday joined the crowd of Silicon Valley companies to acknowledge some stock options awarded to employees might have been mishandled -- a problem that threatens to raise questions about the accuracy of past financial statements.

Without providing details, the Cupertino, Calif.-based maker of computers and the iPod music player said its own internal investigation had uncovered "irregularities" in employee stock options issued between 1997 and 2001.

The troublesome stock options included a batch that went to Apple co-founder Steve Jobs, but the company said those rewards were canceled before its chief executive officer could cash them in and realize a gain.

Apple said it has hired an outside lawyer to spearhead an investigation and has notified the Securities and Exchange Commission of the problem.

The SEC has subsequently launched its own inquiry after other companies have raised a red flag about their past stock options. The Justice Department also has subpoenaed information from many companies suspected of rigging its stock options to lock in bigger windfalls for top executives and other employees.

Nearly half of the 57 companies that have disclosed stock option trouble are based in Silicon Valley and other parts of the San Francisco Bay area.

With yesterday's disclosure, Apple becomes the best known of the lot with a stock option cloud hanging over it.

"Apple is a quality company, and we are proactively and transparently disclosing what we have discovered to the SEC," Mr. Jobs said. "We are focused on resolving these issues as quickly as possible."

The news, released after the stock market closed, appeared to jar investors. Apple shares surged $2.95, or 5.3 percent, to close at $58.97 on the Nasdaq Stock Market, then retreated by $1.87 in extended trading.

Most of the stock option investigations so far have revolved around a practice known as "backdating."

This occurs when a handful insiders retroactively decide to set a stock option's exercise price at an ebb in a company's stock price instead of pegging the exercise price to the prevailing market value at the time of the award.

Stock options become more valuable as the market price rises above the exercise price, so backdating fattens the recipient's profit.

Backdating stock options isn't necessarily illegal, but it can cause a company to improperly deduct employee compensation expenses -- a misstep that could exaggerate profits and result in an underpayment of taxes.

If the backdating isn't properly disclosed, regulators also might interpret the action as a form of financial fraud, exposing companies to civil penalties and a raft of shareholder lawsuits.